1. Field of the Invention
Embodiments disclosed herein relate generally to a power converter, and more specifically, to a power converter having a switch controller that adaptively sets the frequency of the switching cycles of the power converter for optimal dynamic load response (DLR).
2. Description of the Related Arts
Switching power converters typically require error circuitry that provides an “error” signal between the output voltage of the power converter and a reference voltage, in order to regulate the output voltage. The error circuitry provides an error signal indicative of a magnitude and polarity (positive or negative) of the output voltage relative to a reference voltage. The error signal allows the power converter to properly regulate the output voltage by increasing or decreasing the amount of power delivered to the output of the power converter in response to the error signal.
Conventional power converters typically generate an error signal by sensing the output voltage as an analog value and deriving the difference between the sensed output voltage and the reference voltage as an analog value. The difference between the sensed output voltage and the reference voltage is amplified to properly regulate the output voltage based on the amplified signal. Conventional power converters may also use an analog-to-digital converter (A/D converter) to generate the error signal depending upon the control scheme being used in the converters. Other conventional power converters may use analog error amplifiers to generate the error signal.
In many conventional isolated switching power converters, the output voltage is directly sensed on the secondary side of a transformer circuit and is compared to a reference voltage that is generally fixed to a chosen voltage. This allows the output voltage of the power converter to be regulated to a target level based on the comparison.
Alternatively, other conventional isolated switching power converters do not sense the output voltage directly. Rather, these converters sense only signals of the primary side of the transformer circuit of the switching power converter to detect the output voltage level. These primary-only signals are compared to a reference voltage fixed to a chosen voltage so that the output voltage of the power converter is regulated to a target level. These isolated switching power converters are commonly referred to as primary-only feedback converters.
For switching power converters which are used to provide regulated power to portable equipment, such as smart phones and laptop computers, there are three major working modes for the switching power converters: standby mode, charging mode, and operational mode.
The standby mode is when the switching power converter is coupled to the AC mains (i.e., the supply voltage) but is disconnected from the electronic device. Thus, the switching power converter is operating in a low load condition (i.e., no load). During standby mode, the switching power converter must maintain output voltage regulation under the no load condition. Furthermore, the switching power converter is required to minimize internal power consumption in order to meet mandated environmental standards. For example, according to the 5-star energy standard, the maximum allowable standby power consumption for a cell phone charger is typically 30 mW under 230 Vac input voltage.
Pulse frequency modulation is an effective and often used method employed in the standby mode, where the operating frequency of the switching converter is reduced to a standby mode operating frequency in response to the no load condition. New trends in power consumption are requesting ultra-low standby power consumptions, such as less than 10 mW and even less than 5 mW, which require much lower no-load operating frequency to regulate the output voltage.
The charging mode is when the switching power converter is coupled to both the AC mains and the electronic device. Thus, the switching power converter is operating under a load condition. Here, the switching power converter provides regulated power to charge the internal battery of the electronic device without the electronic device being in active use. In this case, there is a one-time “low to high” dynamic load placed on the switching power converter at the time when the electronic device is initially connected to the switching power converter. In this situation, the switching power converter transitions from the standby mode to the charging mode. During the transition, the operating frequency is increased from the standby operating frequency to a higher frequency associated with the load condition. In response to the sudden increase in output load, there is an initial drop of the output voltage as well as overshoot ringing in the output voltage. The amount of output voltage drop and ringing is largely based on the output filter components and the speed of control loop. Once connected, the load placed on the switching power supply is mostly static and changes slowly as the battery charge state gradually increases.
Conversely, when the switching power converter transitions from the charging mode to the standby mode, there is a one-time “high to low” dynamic load placed on the switching power converter. When the high to low dynamic load is detected by the switch controller, the switching power converter is placed in the standby mode with the associated reduction in the switching frequency to the standby operating frequency that corresponds to the no load condition. In this case, there is an associated output voltage rise and overshoot ringing when the high to low dynamic load is placed on the converter.
FIG. 1A illustrates waveforms of a conventional switching power converter when transitioning between the standby mode and the charging mode and vice versa, which is otherwise known as a “one-time” dynamic load response. The one-time dynamic load condition references to a low frequency change of the output load, typically lower than 10 Hz. In particular, FIG. 1A illustrates the output load (IOUT) waveform 101, operating frequency (FSW—FINAL) waveform 103, and the output voltage (VOUT) waveform 105 of conventional switching power converters during the one-time dynamic load response.
For conventional switching power converters which employ primary-only feedback, the feedback signal representing the output voltage of the switching power converter is sensed at each switching cycle. Thus, a limitation to conventional primary-feedback-only switching power converters is that a switch controller of the converter can only respond to load changes switching cycle by switching cycle at the falling edge of the voltage feedback signal. The switch controller controls the operating frequency of the switching power converter upon detection of a change in the output voltage. A change in the output voltage indicates a change in output load. If the switching frequency is low (e.g., 300 Hz) such as in the low load condition, the switching period can be a long time period. This long period is otherwise known as the blank time or blind spot since the primary winding of the transformer does not have any sample information between two switching cycles while the switching power converter is operating and attempting to regulate the output voltage.
In FIG. 1A, the output load waveform 101 illustrates one period of the output load during the one-time “low to high” and “high to load” dynamic load. The output load waveform 101 cycles between a “low” load condition (i.e., no load) and a “high” load condition. The output voltage waveform 105 illustrates the transient response of the output voltage during the one-time dynamic load response. The switching power converter maintains the output voltage at the output voltage setpoint (V-out Setpoint) that represents the steady state output voltage of the converter, but can operate within a maximum allowable output voltage (V-out (MAX)) and a minimum allowable output voltage (V-out (MIN)). The operating frequency waveform 103 illustrates the switching frequency of the conventional switching power converter during the one-time dynamic load response.
When the output load is low 107, the conventional switching power converter is operating at the standby mode operating frequency (e.g., 300 Hz) 109 associated with the standby mode operation or low output load condition. During the low output load condition, the output voltage of the switching power converter is at the output voltage set point (V-out Setpoint). The low operating frequency during the low output load 107 makes the switch controller of the switching power converter less able to detect a rising edge of dynamic load.
In other words, the low operating frequency causes long blind spots representative of a long switching period. The long blind spot results in slow dynamic load response when the output load transitions 113 from a low load condition 107 to a high load condition 121. Depending on when the transition 113 from the low load 107 to high load condition 121 occurs in relationship to the blind spots, the output voltage drops (undershoots) 115 because the low operating frequency 109 of the converter prevents the converter from quickly responding to the load change.
Responsive to the output voltage undershoot 115 indicating a transition 113 to the high-output load condition 121, the switch controller increases 117 the operating frequency of the switching power converter to the high-output load operating frequency 119. The high-output load operating frequency 119 is the frequency in which the converter operates during a high-output load 121. Raising the operating frequency to the high-output load operating frequency 121 causes the output voltage to reach the steady state output voltage setpoint 111.
When the output load transitions 123 from a high-output load condition 121 to the low load condition 107, the output voltage rises (overshoots) 125. The rise in the output voltage 125 is detected by the switch controller and indicates a load change back to the low load state 107. The switch controller immediately responds by controlling the switching cycles to deliver reduced energy to the secondary load. Therefore, the switch controller lowers 127 the operating frequency to the standby mode operating frequency 109 in response to the sudden drop in the output load. By lowering the operating frequency to the standby mode operating frequency 109, the output voltage reaches the steady state output voltage setpoint 111.
FIG. 1B illustrates a detailed view of portion 129 of the waveforms in FIG. 1A during the transition 123 from the high load 121 to the low load 107 condition. In addition to the output load waveform 101, output voltage waveform 105, and operating frequency waveform 103 of the switching power converter, FIG. 1B illustrates the driver output waveform 133. The driver output waveform 133 illustrates the output drive signal of the switch converter. The output drive signal controls when a switch (e.g., metal-oxide-semiconductor field effect transistor) of the switching power converter is turned on or turned off.
As previously described, during the high output load 121, the output voltage is at the output voltage set point 111 and the switching power converter is operating at the high load operating frequency 119 (FSW at High Load Steady State). During the time period 135 when the converter is operating at the high load operating frequency 119, the output drive signal is outputted at a higher frequency compared to when the converter is operating at the standby mode operating frequency (FSW at No-Load Steady State) 109.
As shown in FIG. 1B, during the high load to low load transition 123, the output voltage rises (overshoots) to the high limit 137 which is higher than the output voltage set point 111 but below the maximum allowable output voltage. Upon detection of the overshoot voltage 137, the operating frequency (FSW—FINAL) is reduced 139 from the high-load operating frequency 119 and eventually settles to the standby mode operating frequency 109 associated with the low-load steady state condition 107. The standby mode operating frequency 109 during the low-load steady state condition 107 allows for ultra-low power consumption that is less than 30 mW or even less than 10 mW. However, due to the response characteristics of the conventional feedback control loop, the operating frequency drops below 141 the standby mode operating frequency 109 for a period of time 143 as shown in FIG. 1B. Note that the reduction of the operating frequency 139 is reflected in the driver output waveform 133. The output control signal is generated less frequently during the time period 147 where the operating frequency is being reduced 139.
The operating frequency eventually reaches the steady state standby mode operating frequency 109 at time T1. If the next low load to high load transition 145 occurs after T1 at time T2, the output voltage drops to the lower limit 149 which is above the minimum allowable output voltage of the converter. The switch controller detects the output voltage reaching the lower limit 149 (i.e., the detection point) of the dynamic load response requirement and accordingly increases 151 the operating frequency to the high load operating frequency 119. However, as will be discussed below, if the change in output load is repetitive, the low load to high load transition 145 may occur when the switching power converter is operating at a frequency 141 below the standby mode frequency 109, resulting in the output voltage undershoot exceeding the lower limit 149 of the dynamic load response requirement.
As discussed above, there are three major working modes for the switching power converters: standby mode, charging mode, and operational mode. The operational mode describes when the switching power converter is coupled to both the AC mains and the electronic device, and the electronic device is in active use. Here, the switching power converter provides regulated power to charge the internal battery of the electronic device and for the active use of the electronic device. In this case, there is a one-time “low to high” dynamic load placed on the switching power converter at the time when the electronic device is initially connected. However, because the electronic device is in active use, repetitive dynamic loads are placed on the switching power converter even after the one-time “low to high” dynamic load transition.
For example, LED elements are frequently used for backlighting of LCD displays commonly used in the electronic device. Pulse width modulation of the LED elements is typically used to provide dimming control. In this case, the LED elements are switched at frequencies typically in the 100 Hz to 200 Hz range. This switching causes a high frequency repetitive dynamic load on the switching power converter. When this occurs, conventional switch controllers are unable to determine if a dynamic load is either a one-time load or a repetitive dynamic load.
At each cycle of the repetitive dynamic load, the switch controller responds to the falling edge of the load by returning the switching power converter to the standby mode. When the subsequent rising edge of the load occurs, the switch controller in conventional switching power converters is thus operating at a lower operating frequency associated with the standby mode. Because of the lower operating frequency, the switch controller cannot quickly respond to the sudden increase in load.
FIG. 2A illustrates waveforms of a conventional switching power converter during repetitive dynamic load changes. The repetitive dynamic load change describes a high frequency output load change from low-output load to high-output load and vice versa, typically higher than 100 Hz. FIG. 2A illustrates the output load (IOUT) waveform 201, operating frequency (FSW—FINAL) waveform 203, and the output voltage (VOUT) waveform 205 of the conventional switching power converter during repetitive dynamic load changes.
In FIG. 2A, the output load waveform 201 illustrates multiple periods of the output load during the repetitive dynamic load. The output voltage waveform 205 illustrates the transient response of the output voltage during the repetitive dynamic load response. Similar to FIG. 1A, the switching power converter attempts to maintain the output voltage at the output voltage setpoint (V-out Setpoint) but can operate within a maximum allowable output voltage (V-out (MAX)) and a minimum allowable output voltage (V-out (MIN)). The operating frequency waveform 203 illustrates the switching frequency of the conventional switching power converter during the repetitive dynamic load response.
When the output load is low 207, the switching power converter is operating at the standby mode operating frequency 209 associated with the low-output load. During the low output load condition, the output voltage of the switching power converter is at the output voltage set point (V-out Setpoint). During the transition 213 from the low-output load 207 to high-output load 215, the output voltage drops (undershoots) 217 since the converter is operating at the standby mode operating frequency 209 which prevents the converter from quickly responding to the load change.
Responsive to the output voltage undershoot 217, the switch controller increases 219 the operating frequency to the high-output load operating frequency 221. When the output load transitions 223 from a high-output load 215 to the low-output load 207, the output voltage rises (overshoots) 225. The rise 225 in the output voltage is detected by the switch controller and indicates a load change. The switch controller immediately responds by lowering 227 the operating frequency to the standby mode operating frequency 209 in response to the sudden drop in the output load.
During repetitive dynamic load conditions, such as the subsequent transition 243 from the low-output load 207 to high-output load 215, the output voltage VOUT will experience high output voltage transients during the rising and falling edge of the output load IOUT as shown in the output voltage waveform 205. Specifically and as will be explained in more detail below with reference to FIG. 2B, the output voltage VOUT experiences voltage undershoot 229 during the transition 243 from a low-output load 207 to high-output load 215 which exceeds the minimum allowable output voltage range (V-out (MIN)), thereby creating control loop instability issues and/or causing malfunction or even damage to the electronic load that is connected to the switching power converter.
FIG. 2B illustrates a detailed view of portion 233 of the waveforms in FIG. 2A during repetitive dynamic load changes. Similar to FIG. 1B, FIG. 2B also illustrates the driver output waveform 235 in addition to the output load waveform 201, output voltage waveform 205, and operating frequency waveform 203 of the switching power converter. During the high-output load 215, the output voltage VOUT is set to the output voltage setpoint 211 and the switching power converter is operating at the high load operating frequency 221 (FSW at High Load Steady State). During the time period 237 when the converter is operating at the high load operating frequency 221, driver output control signal is outputted at a higher frequency compared to when the converter is operating at the standby mode operating frequency (FSW at No-Load Steady State) 209.
After the transition 223 from the high-output load 215 to the low output load 207, the output voltage rises (overshoots) to the high limit 241 which is higher than the output voltage setpoint 211, but below the maximum allowable output voltage. Upon detection of the overshoot voltage 241, the operating frequency (FSW—FINAL) is reduced 239 to bring the output voltage to the steady state output voltage setpoint 211. During the reduction 239 of the operating frequency, the frequency falls below the steady state low-load operating frequency 209 between time periods T0 and T1. The reduction of the operating frequency 239 is reflected in the driver output waveform 235. The output control signal is generated less frequently during the time period 255 when the operating frequency is being reduced 239 compared to when the converter was operating at the high load operating frequency 221 during time period 237.
Because of the repetitive load changes, the subsequent transition 243 from low load 207 to high-load 215 occurs at time T0 when the operating frequency is below 245 the standby mode operating frequency 209 for the low load steady state due to the response characteristics of the conventional feedback control loop used in the converter. That is, the transition 243 occurs during the blank time 247. Because of the lower operating frequency, the blank time 247 is much longer because the primary-feedback-only switching power converter can only respond to the load changes switching-cycle-by-switching cycle. Thus, the next control loop response can only occur at the next switching cycle 249 which corresponds to the end of the blank time 247. Because of the long blank time, the output voltage VOUT experiences deep undershoot 251 until the end of the blank time 249 is reached at T1 where the operating frequency is increased 253 to the high load operating frequency 221 resulting in the output voltage reaching the output voltage set point 211. The deep undershoot 251 can result in malfunction and even damage to the electronic load that is connected to the switching power converter during the repetitive dynamic load condition since the switching power converter cannot respond to the voltage undershoot 251 until the next switching cycle of the driver output.